Near-field radiation monitor

ABSTRACT

A probe having a pair of thin film thermocouples positioned in quadrature functions as both an antenna and detector. The thermocouple elements develop a voltage commensurate with the radiofrequency power of the impinging field and this voltage is then used to drive a meter element and display a quantity directly readable as radiofrequency power.

United States Patent Aslan Feb. 8, 1972 [54] NEAR-FIELD RADIATIONMONITOR [21] Appl. No.: 848,620

[52] US. Cl ..325/363, 325/367, 324/106,

3,154,736 10/1964 Howard ..324/l06 3,182,262 5/1965 Schumann .....325/673,237,101 2/1966 Vaughan..... ...324/l06 3,384,819 5/1968 Rinkel 324/1063,450,992 6/1969 Holland ...324/106 3,056,926 10/1962 Borck et al...325/67 3,109,988 11/1963 Hoover ..325/367 Primary Examiner-Robert L.Grifiin Assistant ExaminerJames A. Brodsky Att0rneyJames A. Eisenman andRobert R. Strack 250/83 R [51] Int. Cl. ..H04b 1/06, GOlr 5/22 [57]ABSTRACT Search ..32 7, 63, 67; 24 106, [58] Field of 5/6 3 3 R A probehaving a pair of thin film thermocouples positioned in quadraturefunctions as both an antenna and detector. The H "5 thermocoupleelements develop a voltage commensurate with [56] Retell Cited theradiofrequency power of the impinging field and this volt- UNITED STATESPATENTS age is then used to drive a meter element and display a quantitydirectly readable as radiofrequency power. 1,643,582 9/ 1927 Martin..324/106 UX 2,365,207 12/1944 Moles ..324/95 12 Claims, 4 DrawingFigures I I3 1 1' 0 49 O j 7 IO I 0 21 42 I6 I5 22 g' i 'lil'lllllll I71I I I/ 1 I I I I I I PATENTEDFEB 8 m 3.641.439

39 INVENTOR EDWARD E. ASL AN diam/07m; cord ATTORNEYS NEAR-FIELDRADIATION MONITOR BACKGROUND OF THE INVENTION 1. Field of the InventionThis invention relates to high-frequency power density measurements; andmore particularly relates to the measurement of high-frequency, ormicrowave, power density with portable instruments.

The increasing use of microwave energy for such purposes as consumer andindustrial heating ovens, has placed possibly dangerous microwave energysources in close proximity to large numbers of inexperienced people. Itis essential that units utilizing microwave energy include propershielding in order to avoid endangering those in close proximitythereto. In order to continually monitor the effectiveness of anyshielding provided, and in order to initially ensure its properinstallation, radiation detectors must be provided to measure anyleakage radiation that may appear. The necessary measurements of theradiated power should be made in close proximity to the units beingtested and yet the measuring instrument should not perturb the field.Such measurements should preferably be independent of the polarizationof the incident energy field and independent of ambient temperature andinfrared radiation. Furthermore, it is of importance that the monitoringdevice or instrument be completely reliable inasmuch as such radiationis not apparent with the use of the normal human faculties.

2. Description of the Prior Art It has been known that thin filmthermocouple elements may be used to terminate a transmission line. Whenconnected this way, the thermocouple will be heated by an amountproportional to the power dissipated therein. This heating effectcreates a voltage across the thermocouple and the value of the voltageis a direct indication of the amount of power absorbed by thethermocouple. These characteristics of thermocouples have made them abasic element in the measurement of radiofrequency power. 7

Most measurements of microwave power are made within waveguides whereinthe thermocouple can be designed and selectively positioned in order toavoid reflection of the power and wherein the environment of thethermocouples can be carefully controlled. There are no known priordevelopments of thermocouple probes which can be used in free space tomeasure microwave radiation in the near field or Fresnel region, withoutdisturbing the field.

Thermistors or bolometers have also been used to measure radiofrequencypower. In some applications, for example, such elements are positionedin one leg of a bridge circuit so that the power required to maintainthe bridge in balance is an indication of the amount of power absorbedby the thermistor. Such elements are generally not suitable for the typeof monitor contemplated by this invention because their sensitivity isdirectly related to the ambient temperature and this cannot beadequately controlled.

Crystal arrangements have also been employed to indicate power density.It is known that crystals have a square law characteristic which makesthem adequate for monitoring relatively low-power densities. However,the restriction of such elements to low-power applications, coupled witha relatively narrow square law range, makes them unsatisfactory formonitors of the type herein contemplated.

SUMMARY OF THE INVENTION The present invention relates to a completelyportable detector effective to detect and measure microwave radiationleakage of elements such as microwave ovens, heaters, driers, medicalequipment, and the like.

At the present time, 915 MHz and 2,450 MHZ are the frequencies mostcommonly used in the type of equipment mentioned. These frequencies havebeen assigned by the Federal Communications Commission for industrial,medical, and scientific devices and are the only two frequencies beingused for consumer ovens. Obviously any detector must be designed foroptimum operation within a prescribed frequency range and such a rangemay not encompass both of the mentioned frequencies. Thus, the presentinvention provides means for adapting a basic instrument for optimumoperation over a variety of preselected ranges.

An object of the present invention is to provide a portable radiationdetector.

Another object of the invention is to provide a portable radiationdetector having minimum perturbation effects upon the field beingexamined.

A further object of the present invention is to provide a portablemicrowave detector that is operable close to the source of possibleradiation.

Still another object of the present invention is to provide a portablemicrowave detector which is independent of polarization of the incidentenergy field.

Another object of the present invention is to provide a portablemicrowave detector which is substantially independent of ambienttemperature and infrared radiation.

Yet another object of the invention is to provide a portable microwavedetector of high reliability and accuracy and which incorporates meansfor indicating when the unit is not operating properly.

Another object of the invention is to provide a portable microwavedetector easily adaptable to measurement of a variety of frequencies.

Another object of the present invention is to provide means forinserting thermocouples into a microwave field without perturbing thefield.

Still another object of the present invention is to provide an improvedradiation detector comprising a thermocouple connected between thermallyand electrically conductive films wherein the combination functions asan antenna with a thermocouple load element.

In accordancewith one aspect of the invention, there is provide aradiation detector comprising antenna means operative in response to anelectromagnetic field to produce an electric current, and a thin filmthermocouple connected thereto, said connection providing a hot junctionbetween said antenna means and cold junctions at the contact betweensaid thermocouple and said antenna means.

In accordance with another aspect of the invention there is provided aradiation detector comprising a pair of orthogonally disposedthermocouple elements, antenna means projecting from said thermocoupleelements and orthogonally disposed relative to one another, means forpositioning said elements within a microwave field, and indicating meansresponsive to the voltage developed by said thermocouple elements inresponse to the presence of microwave energy.

A more thorough understanding of the invention, along with a betterappreciation of the objects and novel features thereof, will beavailable following a consideration of the following description whichis made in conjunction with the attached drawings.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 illustrates an embodiment ofthe invention comprising a hand-held probe unit and indicating device;

FIG. 2 is a cross-sectional view taken along the longitudinal axis of aprobe embodying the invention;

FIG. 3 is a top view of the sensor assembly adapted for mounting uponthe probe illustrated in FIG. 2; and

FIG. 4 is an elevation view of a cross section taken through the sensorassembly when only one thermocouple is mounted thereon.

DESCRIPTION OF THE PREFERRED EMBODIMENT FIG. 1 illustrates the majorcomponents of a radiation detector embodying the present invention.These components include a hand-held probe 10 having an antenna orspacer member 13. The probe is connected by a coaxial cable 11 to anelectronic voltmeter 12. Typically, probe 10 may be tubular in shape, 12inches long, and approximately three-fourths of an inch in diameter.Spacer 13 is made of material having free space characteristics and isgenerally designed to permit positioning of the end of probe 2 inchesfrom the source of radiation. It is conical in configuration; the rearsurface being perpendicular to its axis and being adapted under certainconditions to mount extending antenna portions.

The cross-sectional view of probe 10, shown in FIG. 2, illustrates theinterconnection of the thermocouple elements to the coaxial cable 16 atthe front end of the probe. The head of the probe includes a projection22 adapted to serve as an anchoring or mounting element forspaceradapter 13. This mounting element may preferably be a projectionfrom cap portion 21 which encircles the barrel 15. The thermocoupleelements 23, 24 make up the sensor assembly 25 which is illustrated inmore detail in FIGS. 3 and 4. Lead sockets 42 couple sensor assembly 25to four separate conductors of a cable 16. Ferrite beads isolate RF fromcable 16. A half wavelength from the sensor assembly, four RF bypassfeed through capacitors 19 interconnect cable 16 with output cable 11.Thus, cable 11 presents to the electronic voltmeter 12, a voltageproportional to the RF power impinging upon the sensor assembly.

The handle of probe 10 includes an outer tubular element 18 and an innercoaxial tube of lossey ferrite material surrounding cable 11. Typically,this surrounding tube 17 offers 30 db. of attenuation at the expectedfrequency of probe operation. It will be seen that the lead wires 1 lwhich carry the direct current output of the thermocouple are shieldedwith aluminum or other suitable materials and further that they aremaintained perpendicular to the plane of the sensor assembly 25. Thisorientation of the lead wires makes them essentially invisible to thepropagated wave when the antenna is placed parallel to the phase front.

With reference to FIG. 3, it will be noted that the sensor assemblyincludes orthogonally disposed thermocouple elements 23 and 24 connectedto antenna conductor strips 26, 27, and 28, 29, respectively. Theantenna strips are in turn mounted upon a suitable substrate 39 which isadapted for mounting on the end of the probe 10. The particular mannerof mounting is not germane to the invention, although the assembly mustbe substantially orthogonal to the axis of the probe. In order to reducethe potential measuring discrepancies arising from the thermocouplesbeing subjected to different ambient temperatures, they are positionedin the same ambient environment. In a particular embodiment, the hot andcold junctions are separated by a distance of0.0 l 5 inch.

FIG. 4 shows in somewhat greater detail the actual assembly of a singlethermocouple and the manner in which it is mounted upon substrate 39.The substrate may be made up of a number of suitable materials, phenolicor glass filled epoxy having been found to be acceptable. Antenna strips26 and 27 are either deposited or otherwise mounted upon the substrate39 in conventional fashion. A silver paint, or the like, 32, 33, maythen be applied adjacent the inner edges of an aperture formed withinthe substrate 39 and on top of this conducting silver paint, silverfilms 34 and 35 are positioned. The function of the silver films 34 and35 is to provide for the mounting of the actual thermocouple elementwhich may comprise, for example, antimony and bismuth strips 37 and 38,respectively. It has also been found that an upper layer 36 of Kapton orthe like provides desirable structural and electrical characteristics.The illustrated assembly is then secured in position and connected totwo of the leadout wires forming cable 16 by means of connectors and 31.The second thermocouple, of substantially identical construction, ismounted orthogonally to that shown in FIG. 4 and thereby completes thegeneral assembly illustrated in FIG. 3.

It is important to note that the dimensions in the drawing have beenchosen for clarity of illustration and should not be used to suggest therelative sizes of the various elements and thin films. The constructionof the illustrated sensor assembly permits the evaporation of allelements into position, in order to create a structure which willfunction as an antenna (i.e., films 34, 35) terminated by a load (i.e.,thermocouple 23) that is also a detector.

In accordance with the understanding of those in this field of activity,it will be appreciated that each thermocouple 23, 24 defines in effect ahot junction at the junction between the two resistive thin film stripsof dissimilar metals 37, 38. Thermally and electrically conductiveresistive strips 34. 35 are adapted for positioning within the field tobe monitored in order to absorb power therefrom and effect heating ofthe hot junction. The voltage developed at the hot junction isproportional to the difference in temperature between it and the coldjunctions formed at the connections between strips 37 and 34, andbetween strips 38 and 35. Since the hot and cold junctions can beclosely spaced, ambient temperature conditions can be virtually ignored.

It is possible to control sensitivity of the detection unit by varyingthe substrate and the dimensions of the hot junction of thethermocouple. The particular frequency being monitored determines thelength of the antenna which is a small fraction of the wavelength atthat frequency. By maintaining the antenna of small size, the unit iscapable of measuring and monitoring power density with a minimum offield perturbation. In one embodiment of the invention, wherein theprobe is used for measuring energy in the 2,450 MHzrange, the antennastrips have a combined length of approximately three-fourths of an inch.The conical spacer-antenna 13 is attachable to the end of probe 10 inorder to provide accurate spacing from a source of radiation leakage andsimultaneously provides mounting means for antenna extensions. Thus, forfrequencies in the 915MHz range, the rear face of cone 13 containsorthogonally disposed conductive strips which connect to strips 26-29 ofsensor assembly 25 and provide an antenna length of approximately 2inches. Suitable pin-type connectors may be used for this purpose.

The direct current outputs from the thermocouples 23, 24 are connectedin series to the electronic voltmeter 12. Since the thin film elementson the sensor assembly 25 are perpendicular to each other, the totaldirect current output voltage is independent of probe orientation andfield polarization. Each antenna is terminated in an element thatproduces a direct current output proportional to the square of theelectric field intensity component parallel thereto. The sum of theseoutputs is proportional proportional the power density and independentof orientation because of this :square law characteristic of thethermocouple. Since it is known that the proportionality constantbetween field intensity and power density is 377 ohms in a far field,this constant is employed to calibrate the output in terms of powerdensity. All probes are calibrated in a far field and the electronicvoltmeter 12 may thus be adapted to read field density in mw./cm.'*.

As previously noted, instruments of the nature herein contemplated, mustat all times be reliable because the human faculties are unable to sensethe power being measured. With the radiation monitor of the presentinvention, a very simple fail-safe technique may be employed. It iscontemplated that a small constant direct current will be connected andpassed through the thermocouple elements at all times. The resultantvoltage developed in the thermocouple is balanced out at the input tothe voltmeter so that no significant reading appears thereon. Using thisconstantly applied power, in the event that excessive power is appliedto the antenna during normal operation, thereby destroying thethermocouple, the voltage applied to the voltmeter will go up,indicating a full-scale meter reading and, if desired, sounding analarm.

It is important to note that because of the unfiltered supply voltagesfrequently used on magnetrons and due to the rotation of stirrers inmicrowave ovens, peak to average power ratios are in the order of 10:].Such peak power pulses have been found to be approximately 8milliseconds long. It is possible that such power pulses may persist fora sufficiently long period to burn out the thermocouple elements.

In order to overcome this problem, the time constant of the elements issuitably chosen. An increase of the surface area of the thermocouple incontact with the substrate has been found to be an effective way ofeffecting this time constant selection. Changing the conductivity of thesubstrate or thickness without changing the geometry of the element doesnot change the time constant. A second method of increasing the timeconstant is by depositing a passive resistive film terminating thedipole on one side of the substrate. This film is essentially matched tothe dipole, and in a particular example, exhibited a resistance on theorder of 100 ohms, so that the maximum RF power would be dissipated inthe element. It is advantageous to make the element of ahigh-temperature melting material so that it can absorb large amounts ofenergy without damage. On the opposite side of the substrate there ismounted another dipole with a very low-resistance thermocouple so thatvery little RF energy is dissipated therein. This second thermocouplewill be heated by the energy in the passive element. The time constantis dependent on the thickness of the substrate, i.e., the time requiredfor the heat to pass from the passive element through the substrate intothe active thermocouple element. The direct current output of thethermocouple is then connected to the meter for power indicationpurposes.

In the event that faster time constant elements are desired so that peakpower density may be measured, the width of the thermocouple element maybe decreased. in this case, the meter may be designed to indicate peakas well as average power as a switched function.

A specific radiation monitor and probe assembly has been described. Itwill be appreciated that variations and modifications in this radiationdetector may be conceived by those skilled in the art. All suchvariations and modifications coming within the scope of the abovedisclosure and the appended claims, are intended to be covered by theseclaims.

What is claimed is:

l. A radiation detector comprising antenna means operative in responseto an electric field to produce an electric current, including thermallyand electrically conductive films forming a dipole; and a thin filmthermocouple connected as a load to said antenna means, the hot junctionof said thermocouple being formed by overlapping end portions of thinresistive strips of dissimilar metal films having a thickness that issmall relative to the skin depth of the wave energy of said electricfield, and the cold junctions of said thermocouple being formed byoverlapping the other end portions of said thin resistive strips withsaid thermally and electrically conductive films; said thermocouple andsaid antenna means being substantially disposed within a plane.

2. A radiation detector according to claim I, wherein said conductivefilms forming a dipole are a small fraction of a wavelength from end toend for the midfrequency of the range of frequencies to be detected.

3. A radiation detector according to claim 1, comprising second antennameans operative in response to an electric field to produce an electriccurrent, and a second thin film thermocouple connected as a load to saidantenna means and effecting cold junctions at the points of connection.said second antenna means and second thermocouple being disposed itright angles to the first-mentioned antenna and thermocouple andsubstantially within said plane.

4. A radiation detector according to claim 1, including means couplingsaid thermocouple to a meter responsive to I the amount of voltagegenerated therein.

5. A radiation detector according to claim 24, wherein said meanscoupling the thermocouple to said meter are positioned substantiallyorthogonally to said conductive films.

6. A radiation detector according to claim 5, wherein said meanscoupling the thermocouple to said meter include bypass capacitorslocated approximately one-half wavelength from said thermocouples.

7. A radiation detector according to claim 5, wherein said thermocoupleelement is mounted at the end of elongated means and includingconnecting means at said end for securing additional conductive means inelectrical contact with each of said first mentioned conductive filmsand being aligned therewith.

8. A radiation detector as defined in claim 1, including nonperturbingmeans for introducing said antenna means and thermocouple into thenear-field region of said electric field, said nonperturbing means beingsubstantially parallel to the direction of field propagation when saidthermocouple and antenna means are aligned with said electric field.

9. A radiation detector as defined in claim 1, including spacing meansof predetermined length and having free space characteristics, adaptedto be interposed between said antenna means and the source of saidelectric field.

10. A radiation detector as defined in claim 9, wherein said antennameans are mounted on said spacing means.

1 1. A radiation detector as defined in claim 10, wherein said spacingmeans and said antenna means are mounted on a unit of predeterminedlength wherein connections are made to said antenna means via conductorsmaintained perpendicular thereto.

12. A radiation detector as defined in claim 10, wherein said spacingmeans is removably connected to said detector and including furtherspacing means having free space characteristics connectable to saiddetector, and further means mounted on said further spacing meansadapted to respond to an electric field of different frequency.

UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTWN Patent No. 36Hl l39Dated February 8 1972 Inventor(s) Edward E. Aslan It is certified thaterror appears in the above-identified patent and that said LettersPatent are hereby corrected as shown below:

Column 2, line 37 Change "vide" to" -vided.

Column H, line '46 Delete "proportional" second occurrence andsubstitute therefor -to.

Column 6, line 15 Change "claim 2%" to claim Signed and sealed this 28thday of November 1972.

(SEAL) Attest:

EDWARD MQFLETCEER TR. ROBERT GOTTSCHALK Attesting Officer Commissionerof Patents FORM PO-105O (10-69) USCOMM-DC 6037 -P69 fl U.S. GOVERNMENTPRINI'ING OFFILE i969 O366334

1. A radiation detector comprising antenna means operative in responseto an electric field to produce an electric current, including thermallyand electrically conductive films forming a dipole; and a thin filmthermocouple connected as a load to said antenna means, the hot junctionof said thermocouple being formed by overlapping end portions of thinresistive strips of dissimilar metal films having a thickness that issmall relative to the skin depth of the wave energy of said electricfield, and the cold junctions of said thermocouple being formed byOverlapping the other end portions of said thin resistive strips withsaid thermally and electrically conductive films; said thermocouple andsaid antenna means being substantially disposed within a plane.
 2. Aradiation detector according to claim 1, wherein said conductive filmsforming a dipole are a small fraction of a wavelength from end to endfor the midfrequency of the range of frequencies to be detected.
 3. Aradiation detector according to claim 1, comprising second antenna meansoperative in response to an electric field to produce an electriccurrent, and a second thin film thermocouple connected as a load to saidantenna means and effecting cold junctions at the points of connection,said second antenna means and second thermocouple being disposed itright angles to the first-mentioned antenna and thermocouple andsubstantially within said plane.
 4. A radiation detector according toclaim 1, including means coupling said thermocouple to a meterresponsive to the amount of voltage generated therein.
 5. A radiationdetector according to claim 24, wherein said means coupling thethermocouple to said meter are positioned substantially orthogonally tosaid conductive films.
 6. A radiation detector according to claim 5,wherein said means coupling the thermocouple to said meter includebypass capacitors located approximately one-half wavelength from saidthermocouples.
 7. A radiation detector according to claim 5, whereinsaid thermocouple element is mounted at the end of elongated means andincluding connecting means at said end for securing additionalconductive means in electrical contact with each of said first mentionedconductive films and being aligned therewith.
 8. A radiation detector asdefined in claim 1, including nonperturbing means for introducing saidantenna means and thermocouple into the near-field region of saidelectric field, said nonperturbing means being substantially parallel tothe direction of field propagation when said thermocouple and antennameans are aligned with said electric field.
 9. A radiation detector asdefined in claim 1, including spacing means of predetermined length andhaving free space characteristics, adapted to be interposed between saidantenna means and the source of said electric field.
 10. A radiationdetector as defined in claim 9, wherein said antenna means are mountedon said spacing means.
 11. A radiation detector as defined in claim 10,wherein said spacing means and said antenna means are mounted on a unitof predetermined length wherein connections are made to said antennameans via conductors maintained perpendicular thereto.
 12. A radiationdetector as defined in claim 10, wherein said spacing means is removablyconnected to said detector and including further spacing means havingfree space characteristics connectable to said detector, and furthermeans mounted on said further spacing means adapted to respond to anelectric field of different frequency.